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Tevatron particles shed light on antimatter mystery

By Valerie Jamieson

WHY the universe is filled with matter rather than antimatter is one of the great mysteries in physics. Now we are a step closer to understanding it, thanks to an experiment which creates more matter than antimatter, just like the early universe did.

Our best understanding of the building blocks of matter and the forces that glue them together is called the standard model of particle physics. But this does a poor job of explaining why matter triumphed over antimatter in the moments after the big bang.

The standard model has it that matter and antimatter were created in equal amounts in the early universe. But if that was the case they should have annihilated in a blaze of radiation, leaving nothing from which to make the stars and galaxies. Clearly that didn’t happen.

A quirk in the laws of physics, known as CP violation, favours matter and leaves the universe lopsided. The standard model allows for a small amount of CP violation but not nearly enough to explain how matter came to dominate. “It fails by a factor of 10 billion,” says Ulrich Nierste, a physicist at the Karlsruhe Institute of Technology in Germany.

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Now researchers at DZero, an experiment at the Tevatron particle accelerator at Fermilab in Batavia, Illinois, have found the largest source of CP violation yet discovered. It comes courtesy of particles known as Bs mesons (arxiv.org/abs/1106.6308).

These are unusual particles because they can transform into their own antiparticle and back again, says Guennadi Borissov, a member of the DZero team based at Lancaster University, UK. That makes them perfect for studying CP violation.

Last year, the DZero experiment studied collisions between protons and antiprotons that create Bs mesons, which then decay into muons. Sure enough, the team found more muons than antimuons, signalling that more matter is created than antimatter.

However, particle physics is littered with findings that disappear as more data is collected. Now Borissov and his colleagues have repeated the study using data from 50 per cent more collisions and the new result boosts the original conclusion (Physical Review Letters, DOI&colon; 10.1103/PhysRevLett.105.081801). “The most likely interpretation is an anomalously high CP violation,” says Guy Wilkinson at the University of Oxford.

Admittedly more work is needed to explain why the universe is filled with matter. “This result won’t explain all of the matter-antimatter asymmetry,” says Val Gibson at the University of Cambridge, “but it could indicate new physics.”

“This result won’t explain all of the matter-antimatter asymmetry but it could indicate new physics”

Several ideas of what this new physics might be are on the table, including so-called supersymmetric particles. So far, the world’s most powerful accelerator, the Large Hadron Collider at CERN near Geneva, Switzerland, has failed to find signs of supersymmetry and this is starting to worry some theorists. But the finding from DZero may turn out to be the pointer they are looking for. “Supersymmetry can easily explain this measurement,” says Nierste.

DZero may not be able to say much more about the lopsided universe, though. The Tevatron is due to shut down in September and DZero has now analysed the majority of its Bs meson data. However, an experiment at the LHC, called LHCb, is ideally suited to studying the Bs meson and particles like it. “LHCb has already taken enough data to be competitive with Fermilab,” says Gibson, who works on the experiment. Her team hopes to report its own results at a conference in Mumbai, India, in August.